Yersinia enterocolitica is a food borne pathogen of humans and animals. Bacteria entering the host cross the epithelial cell layer in the terminal ileum of the small intestine, accessing the reticuloendothelium system below. The ail gene of Y. enterocolitica has been shown to have a role in the ability of this organism to enter tissue culture cells and to resist killing by human serum in infection has not been determined. The investigators identified a locus involved in regulating expression of the ail gene in response to temperature and growth phase. This locus has homology to the clpPX operon of Escherichia coli. ClpP associates with ClpX, or one of several other ClpX homologs, to form an ATP dependent protease. The Clp protease, and homologous proteases from other organisms, have been demonstrated to affect gene expression by degranulation of regulatory proteins. It is likely the product of this locus will regulate ail gene expression indirectly. The clpP gene has been implicated in virulence of Salmonella typhimurium, perhaps through its effects on the expression of important virulence proteins. Experiments proposed here will determine the role of these interesting regulatory proteins in the regulation of ail gene expression. In addition, the actual regulator of ail gene expression that is controlled by the clp locus will be identified. The investigators have identify three additional loci that regulate ail gene expression independently of the Clp protease in response to temperature and growth phase. The genes mapping to these different loci will be characterized to learn more about these regulators of ail gene expression. Completion of these studies will contribute to their understanding about the role of the ail gene and its regulators in the virulence of Y. enterocolitica.